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context_tracking: Restore previous state in schedule_user

It appears that some SCHEDULE_USER (asm for schedule_user) callers
in arch/x86/kernel/entry_64.S are called from RCU kernel context,
and schedule_user will return in RCU user context. This causes RCU
warnings and possible failures.

This is intended to be a minimal fix suitable for 3.18.

Reported-and-tested-by: Dave Jones <davej@redhat.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Frédéric Weisbecker <fweisbec@gmail.com>
Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

authored by

Andy Lutomirski and committed by
Linus Torvalds 7cc78f8f ebcd241a

+6 -2
+6 -2
kernel/sched/core.c
··· 2874 2874 * or we have been woken up remotely but the IPI has not yet arrived, 2875 2875 * we haven't yet exited the RCU idle mode. Do it here manually until 2876 2876 * we find a better solution. 2877 + * 2878 + * NB: There are buggy callers of this function. Ideally we 2879 + * should warn if prev_state != IN_USER, but that will trigger 2880 + * too frequently to make sense yet. 2877 2881 */ 2878 - user_exit(); 2882 + enum ctx_state prev_state = exception_enter(); 2879 2883 schedule(); 2880 - user_enter(); 2884 + exception_exit(prev_state); 2881 2885 } 2882 2886 #endif 2883 2887